Integrated circuit designers have always sought the ideal semiconductor memory: a device that is randomly accessible; can be written to or read from very quickly; is non-volatile, but indefinitely alterable; and consumes little power. Magnetoresistive random access memory (MRAM) technology has been increasingly viewed as offering all of these advantages.
An MRAM memory cell contains a non-magnetic conductor forming a lower electrical contact, a pinned magnetic layer, a barrier layer, a free magnetic layer, and a second non-magnetic conductor. The pinned magnetic layer, tunnel barrier layer, and free magnetic layer are collectively termed the magnetic tunnel junction (MTJ) element.
Information can be written to and read from the MRAM cell as a “1” or a “0,” where a “1” generally corresponds to a high resistance level, and a “0” generally corresponds to a low resistance level. Directions of magnetic orientations in the magnetic layers of the MRAM cell cause resistance variations. Magnetic orientation in one magnetic layer is magnetically fixed or pinned, while the magnetic orientation of the other magnetic layer is variable so that the magnetic orientation is free to switch direction. In response to the shifting state of the free magnetic layer, the MRAM cell exhibits one of two different resistances or potentials, which, as described above, are read by the memory circuit as either a “1” or a “0.” It is the creation and detection of these two distinct resistances or potentials that allows the memory circuit to read from and write information to an MRAM cell.
A bit of information may be written into the MTJ element of an MRAM cell by applying orthogonal magnetic fields directed within the XY-plane of the MTJ element. Depending on the strength of the magnetic fields, which are created by a current passing through the write line, the free magnetic layer's polarization may remain the same or switch direction. The free magnetic layer's polarization then may continue to be parallel to the pinned magnetic layer's polarization, or anti-parallel to the pinned magnetic layer's polarization.
A bit of information is retrieved from the MTJ element by measuring its resistance via a read current directed along the Z-axis, transverse to the XY-plane. The state of the MTJ element can be determined by the read conductor measuring the resistance of the memory cell. The MTJ element is in a state of low resistance if the overall orientation of magnetization in the free magnetic layer is parallel to the orientation of magnetization of the pinned magnetic layer. Conversely, the MTJ element is in a state of high resistance if the overall orientation of magnetization in the free magnetic layer is anti-parallel to the orientation of magnetization in the pinned magnetic layer.
Conventional MRAM structures, such as that depicted in FIG. 1, typically have a write conductor 20 and a read conductor 26, separated by a liner 17, together forming a word line 32. Other layers may be included, but are omitted for clarity. The word line 32 of a conventional MRAM structure is typically formed in a first insulating layer (typically an oxide layer) 10, with an MTJ element 28 formed over the word line 32. Typically, the read conductor 26 is less than 500 nm wide and less than 50 nm thick. The dimensions of the read conductor 26 and the liner 17 separate the MTJ element 28 from the write conductor 20.
Conventional MRAM structures electrically isolate the write conductor 20 from the MTJ element 28 to protect the MTJ element 28 from a voltage created when a current is applied to the write conductor 20 to write a bit of information onto the MTJ element 28. However, by isolating the write conductor 20 from the MTJ element 28, a higher current is necessary to achieve the same electromagnetic field to write a bit of information if the write conductor 20 was not electrically isolated. The higher current results in higher voltages applied to the MTJ element 28.